Ballymahon (Irish: Baile Uí Mhatháin, meaning "Mahon's town") on the River Inny is a town in the southern part of County Longford, Ireland. It is located at the junction of the N55 National secondary road and the R392 regional road.
Ballymahon derives its name from Gaelic Baile Mathuna (Town of Mahon). This may refer to Mahon (Mathgamain mac Cennétig), a southern chieftain and the elder brother of Brian Boru, who is believed by some to have fought a battle in 960 in the vicinity of Ballymahon at Shrule (in Irish Sruaith Fhuil, River of Blood) where he defeated O'Rourke of Cavan and laid claim to lands in the area. This is disputed by others who claim Mahon may relate to a sub chieftain of the O'Farrells who ruled over this part of County Longford in the 14th century.
In comparison to other towns and villages in Co. Longford, Ballymahon is a young town, the centre of activity gradually shifting from the nearby ancient parish church at Shrule. The earliest documentary evidence of Ballymahon was in the year 1578. The map of the 'Down Survey' 1654 by Sir William Petty shows Ballymahon as a group of houses situated at the southern end of the present town, on the right bank of the Inny River. Two main families, the Shuldham family of Moigh House and the King-Harman family of Newcastle House developed the town in the mid-nineteenth century. The buildings in the town are of late Georgian architecture, with two and three storey gabled houses, colour-washed and in rows of three and four. A special feature of the town is its wide main street. During the period 1788-1853 the Roman Catholic Bishops' of Ardagh and Clonmacnoise resided in the town and the parish church served as a Pro-Cathedral until St. Mel's Cathedral was built in Longford.